A light emitting apparatus using a combination of semiconductor light emitting elements and phosphors attracts attention as a next generation light emitting apparatus expected to realize low power consumption, compact size, high intensity, high color gamut, and high color rendition, and is actively researched and developed. Primary light emitted from a light emitting element in a range from the longer ultraviolet to the visible blue, i.e. 380-480 nm, is usually used. A light converter using various phosphors suitable for this application is also proposed.
At present, as a white light emitting apparatus of this type, a combination of a light emitting element emitting blue light (peak wavelength: around 460 nm) and a trivalent cerium-activated (Y, Gd)3(Al, Ga)5O12 phosphor or a divalent europium-activated 2(Sr, Ba, Ca)O.SiO2 phosphor, which is excited by the blue light and emits yellow light, is mainly used. However, color gamut (NTSC ratio) of a color liquid crystal display (LCD) using such a light emitting apparatus is about 70%, although higher color gamut is required in compact LCD.
Furthermore, for the light emitting apparatus of this type, an attempt has recently been made not only to improve luminous efficiency (brightness) but also to increase input energy to achieve higher brightness. When input energy is increased, it becomes necessary to effectively dissipate the heat of the entire light emitting apparatus including the light converter. For achieving this, the development of the entire structure and materials of light emitting apparatus has been pursued. However, a temperature rise in the light emitting element and the light converter during operation is still inevitable.
However, in the trivalent cerium-activated (Y, Gd)3(Al, Ga)5O12 phosphor, in particular, there is a technical problem in that it is impossible to set input energy at high status, because the luminance at 100° C. decreases to about 85%, compared to the luminance (brightness) of 100% at 25° C. Therefore, it is urgently necessary to improve the temperature characteristic of phosphors to be used for the light emitting apparatus of this type.
For these technical problems, it is known that the use of a divalent europium-activated oxynitride green light emitting phosphor which is β-type SiAlON represented by EuaSibAlcOdNe results in a light emitting apparatus having good color gamut (NTSC ratio) and temperature characteristic.
However, the divalent europium-activated oxynitride green light emitting phosphor which is β-type SiAlON is basically a columnar crystal, and as for a group of phosphor particles (phosphor particle group), a phosphor with a value obtained by dividing a longer particle diameter by a shorter particle diameter exceeding five is easily produced. The use of a phosphor particle group including a large number of particles with the value obtained by dividing a longer particle diameter by a shorter particle diameter exceeding five has a technical problem in that when the phosphor particle group is dispersed in a resin, such a phenomenon as aggregation which is assumed to be attributable to the form factor takes place to prevent uniform dispersion, and a good characteristic (brightness) cannot be obtained.
Therefore, it is urgently necessary to develop a divalent europium-activated oxynitride green light emitting phosphor particle which is a β-type SiAlON represented by EuaSibAlcOdNe having a controlled shape, and a high-efficiency light emitting apparatus using the same. Japanese Patent Laying-Open No. 2005-255895 (Patent Literature 1), for example, discloses β-type SiAlON in which a mean value of the aspect ratio (the value obtained by dividing the length of the longer axis of a particle by the length of the shorter axis) is 1.5 or more and 20 or less. However, in the examples of Patent Literature 1, the aspect ratio of the phosphor particle in each example is not disclosed, and no mention is made to the aspect ratio and the characteristic. Furthermore, Patent Literature 1 only shows a columnar shape (photograph) in FIG. 2.
Furthermore, WO2009/008250A1 (Patent Literature 2) discloses a β-type SiAlON and a phosphor particle group characterized in that 60% or more of the phosphor particle group is composed of the phosphor particles with the aspect ratio being greater than 1.0 and not greater than 3.0. Patent Literature 2 discloses that significant enhancement of brightness is achieved, compared to conventional products.
However, more excellent color gamut than that of conventional cold cathode tubes has recently been expected for application to a large LCD such as a thin-screen television. Therefore, further stabilization of the characteristic (fluctuations in brightness and chromaticity) is also desired in the above-noted phosphor particle group and the light emitting apparatus using the same.